1. Field of the Invention
The invention relates generally to global positioning system (GPS) receivers more particularly to a GPS receiver having a fast time to first fix using only an approximate time with information for the velocity of GPS satellites.
2. Description of the Prior Art
The United States government maintains a constellation of earth orbiting satellites for a global positioning system (GPS). The GPS satellites broadcast GPS signals having location-determination information that can be received and decoded by a GPS receiver for determining a geographical location and time. In some applications for GPS receivers it is desirable or required to have a fast time to a first location fix.
GPS receivers determine location by measuring ranges to four or more GPS satellites. These ranges are termed pseudoranges because they include a term caused by a time error of the internal clock in the GPS receiver. The pseudoranges are measured by determining phase offsets between GPS pseudorandom (PRN) codes received in the GPS signals and internal GPS replica PRN codes referenced to the internal clock. The GPS receiver then determines a GPS-based time by monitoring the GPS signal until a Z-count is decoded. The GPS-based time is used to determine the times that the phase offsets were measured. The GPS-based measurement times are then used with ephemeris information that is received in the GPS signals for calculating the instantaneous locations-in-space of several GPS satellites and for linearizing locations equations relating the calculated locations-in-space to the measured pseudoranges. Having four or more linearized equations for four or more GPS satellites, respectively, the GPS receiver can resolve the three dimensions of geographical location of the GPS receiver and correct the error in the internal clock time.
Existing GPS receivers are typically specified as having an intrinsic geographical location accuracy of about twenty meters with no degradation due to selective availability and a location accuracy of about one-hundred meters with selective availability at current levels. In order to achieve these accuracies the GPS receiver must determine or obtain an accurate time either during or before resolving location in order to calibrate the measurement times to GPS-based time. An error in the time calibration causes errors in the calculations of the locations-in-space of the GPS satellites and typically results in an inaccurate location. In most existing GPS receivers, the first accurate location fix requires that accurate GPS time be obtained in the Z-count in the GPS signal. Unfortunately, the Z-count repeats only every six seconds. Further, brief outages caused by foliage or buildings as the GPS receiver is moving can cause dropouts in the GPS signal with the result that one or more Z-counts may be missed before GPS-based time can be determined. Some workers have attempted to eliminate the need for waiting for the Z-count by using an atomic clock for providing an accurate internal clock time or a radio receiver for receiving an external standard time signal. However, such additional hardware can add significantly to the cost of the GPS receiver.
There is a need for a GPS receiver having a fast time to an accurate first fix before receiving a Z-count without the requirement for costly additional hardware.